1. Field of the Invention
The present invention relates to a Coded Orthogonal Frequency Division Multiplex (COFDM) demodulator.
2. Discussion of the Related Art
FIG. 1 illustrates the principle of a COFDM modulation. Data packets to be transmitted are put in the form of N complex coefficients associated with N respective frequencies (or carriers). Number N of the frequencies is equal, for example, to 1,705 for the so-called “2K” mode and to 6,817 for the so-called “8K” mode, in digital television radio transmission. Each complex coefficient corresponds to a vector which is illustrated in FIG. 1 as starting from a frequency axis at a point indicating the frequency associated with the coefficient.
The set of these N coefficients is processed by inverse fast Fourier transform (IFFT), which generates a “symbol” formed of a sum of modulated carriers, each carrier having an amplitude and a phase determined by the associated complex coefficient. The symbol thus generated is transmitted.
Conventionally, in radio transmission, the width of the information channel is 6, 7, or 8 MHz and each carrier is separated from the next one by a frequency difference Δf=1/Tu. Tu is the transmission time of a symbol and is called the operating lifetime. The operating lifetime is on the order of 224 μs in 2K mode and 896 μs in 8K mode, for a 8-MHz passband.
Upon reception, a receiver submits the symbol to the inverse processing, that is, mainly, a fast Fourier transform (FFT) to restore the initial complex coefficients.
The receiver actually receives the signal transmitted by the transmitter as well as a multitude of attenuated and delayed signals originating from different echoes. The information channel, taken by the signal to be demodulated, is then said to be a multiple-path channel. Such a channel has a frequency response which is not flat, but comprises holes and bumps due to the echoes and reflections between the transmitter and the receiver. The channel is said to be of fixed type when it has a frequency response which is substantially constant along time. Conversely, the channel is said to be of time-variable type when it has a frequency response which varies along time.
To determine an estimate of the frequency response of the information channel, regularly distributed vectors P1, P2, P3 . . . having a known constant value are used. These vectors, or the corresponding carriers, are said to be pilots. They are used to reflect the distortions undergone by the transmitted channel and by the information that they provide on the channel response, they enable correcting the unknown vectors located between pilots.
FIG. 2 schematically shows the place of pilots in the symbols. The symbols are gathered in frames of 68 symbols, conventionally in digital television radio transmission (standard ETSI EN 300 744, V1.4.1).
In FIG. 2, each line represents a symbol and each box represents the position of a carrier. The carriers are defined as going from a position 0 to a position Kmax, Kmax being equal to 1,704 in 2K mode and 6,816 in 8K mode. Indeed, a portion only of the possible frequencies is used, especially due to risks of losses at the channel border. The pilots are of two types.
On the one hand, there are, in each symbol, continuous pilots Pc. The continuous pilots correspond to specific frequencies distributed in the channel. In the above-mentioned ETSI standard, there are 45 of these in 2K mode and 177 in 8K mode. Continuous pilots are present in all the symbols and always occupy the same frequency position. In FIG. 2, only the continuous pilots corresponding to positions 0 and Kmax have been shown.
On the other hand, there are, in each symbol, so-called “scattered pilots” Pr, which are arranged every 12 carriers, and shifted by three positions between two successive symbols. Thus, every four symbols, the same arrangement of scattered pilots Pr can be found.
The continuous and scattered pilots, of constant amplitude on transmission, are used to know the frequency and pulse response of the channel. For this purpose, at the receiver, the complex time received signal, after having been put in baseband, is provided to a fast Fourier transform unit providing the symbol in the frequency field. The pilots are sampled from this symbol. An estimate of the frequency response of the channel is determined based on the continuous and scattered pilots sampled from four successive symbols. The estimate of the frequency response is especially used to correct the vectors associated with the carriers located between pilots. The pulse response of the channel, obtained from the inverse Fourier transform of the frequency response estimate, is especially used to finely position a window on which the fast Fourier transform is performed.
Many operating parameters of the COFDM demodulator are generally optimized on operation of the COFDM to improve the demodulator performance.
A method to adjust the operating parameters of the demodulator consists of analyzing the data received after complete decoding and determining bit error rate BER. The previously-described operating parameters of the demodulator can then be modified by trial and error until the bit error rate is acceptable.
Another criterion of the measurement of the demodulation quality corresponds to the signal-to-noise ratio SNR which can be determined on extraction of the continuous or scattered pilots.
Among the operating parameters of the demodulator that can be optimized, some depend on the fixed or time-variable type of the channel. For example, according to the nature of the channel, a specific method can be privileged to determine the channel frequency response, thus enabling more efficiently obtaining a precise estimate of the frequency response of the channel. Further, according to the channel type, the amplifier gains provided at the demodulator input may be set in adapted fashion to amplify the signal received by the demodulator. Further, before performing the fast Fourier transform, the complex signal is generally corrected in frequency and in time by algorithms implementing time constants that can be adjusted according to the type of channel.
However, the method for optimizing the operating parameters of the demodulator using bit error rate BER is relatively inaccurate and slow as concerns the operating parameters that can be optimized according to the type of information channel. Indeed, the complete data decoding must be awaited, which can be very long. A modification of the operating parameters of the demodulator based on such a criterion is thus performed long after a channel variation, and thus generally too late to avoid a data loss. Further, bit error rate BER does not indicate how the operating parameters of the demodulator are to be modified to improve its performances. Indeed, a proper adjustment of the operating parameters of the demodulator will only be acknowledged by a subsequent decrease in the bit error rate.
Like for bit error rate BER, it is not possible to deduce from signal-to-noise ratio SNR the reasons of the degradation of the demodulation. A parameter optimization method implementing signal-to-noise ratio SNR is thus slow and inaccurate since the operating parameters of the demodulator must thus be modified by trial and error until the signal-to-noise ratio is acceptable.